Energy harvesting PV power systems, also referred to as solar power systems, have been used to provide electric power in various applications including residences and airborne and space borne aircraft such as satellites and unmanned aerial vehicles. For residential applications, a solar panel has a relatively small number of cells where each cell is quite large, such as a silicon PV cell that may be about six inches by six inches in area, and there may be approximately 72 such cells within in a single residential solar panel. Each solar cell typically is designed to produce a certain voltage, for example, about 0.6 volts for silicon cells, which has only a weak dependence on the amount of light radiation received at the cell. Such cells may be electrically connected in series within a panel, in order to increase the harvested energy output voltage for example, 40 Volts dc (Vdc). A typical residential solar system may include several such panels, for example between five and ten, providing up to several hundred volts. A dc-ac conversion circuit is then used to obtain the more common 120 Vac output voltage.
For airborne and spacecraft applications, a PV system is used as a primary power system that feeds energy storage devices such as a battery, as well as other components of the aircraft or spacecraft such as the propulsion system. While the battery may have a relatively low voltage of less than five volts, the propulsion system may need several hundred volts at its power supply input. Accordingly, a dc-dc up converter or voltage boost circuit is used to increase, for example, a 40-volt PV output to 800 or even 1000 volts. For space applications or unmanned aerial vehicle applications, it can be seen that a power supply bus is needed that can support low, medium and high voltages depending upon the operational mode of the spacecraft or aircraft. For example, a high voltage is needed for acceleration by the propulsion unit of a satellite during orbital transfers and other maneuvers, whereas a medium voltage is needed for regular operations, and a low voltage is needed for riding out a solar storm or a safe shutdown mode. In addition, reliability, availability and maintenance needs of the spacecraft or aircraft strongly impact the design of the electrical power system, which is a critical component in such applications.